1. Field of the Invention
The present invention concerns a method and a device to process complex image data (i.e., data having a real part and an imaginary part) of an examination subject in a magnetic resonance (MR) data acquisition. The invention in particular concerns such methods and devices to process image data in which the MR system has multiple acquisition units for data acquisition, for example multiple acquisition coils or multiple segments of a coil array, wherein complex image data are acquired at multiple echo times with each of the acquisition units.
2. Description of the Prior Art
MR imaging has found wide application since it enables the acquisition of two-dimensional or three-dimensional image data that can depict structures inside an examination subject with high resolution. In MR imaging, the nuclear spins of hydrogen nuclei in an examination subject are aligned in a basic magnetic field (B0) and subsequently excited by the radiation of RF (radio-frequency) pulses. The excited magnetization is detected as a function of time, with a spatial coding being achieved by various known methods. The acquisition of the signals frequently takes place with a quadrature detection so that both the phase and the amplitude of the signal are detected. The signals detected in positional frequency space (k-space) can accordingly be represented as complex numbers and be transformed into image space by means of a Fourier transformation in which phases and amplitudes can now be determined with spatial resolution.
In many imaging methods, only the amplitude of the complex image data is used to create an intensity image. The phase information is discarded.
Phase values can deliver valuable information that, for example, can be used in the determination of an adipose tissue or aqueous proportion. Some imaging methods use the phase information. Examples of such methods include phase contrast imaging and proton resonance frequency (PRF) shift thermometry. In PRF shift thermometry, a phase shift in acquired phase images is detected that is caused by a temperature dependency of the proton resonance frequency.
In addition to phase shifts with information content, such as a temperature-dependent phase shift of a proton spin resonance frequency, there are effects that can cause unwanted phase shifts and can lead to artifacts. Causes of such unwanted phase shifts can be, for example, a system-dependent inhomogeneity of the basic field B0, the susceptibility of articles and materials within or in proximity to the examination subject, phase shifts of the radiated RF pulses, and errors in the chronological order of the acquisition sequence. Such phase shifts make it difficult to compare and/or to combine with one another image data acquired at different echo times. Different phase shifts can also occur in different acquisition channels, which can hinder a combination of image data that are associated with different echo times and different acquisition coils.
Various approaches have been described to combine complex image data that were acquired with multiple acquisition units. U.S. Pat. No. 6,483,308 B1 and U.S. Pat. No. 7,227,359 B1 are examples. In these approaches, for each acquisition coil the image data that are acquired with this acquisition coil at different echo times are combined. A merging of the combined images that were acquired for the different acquisition coils subsequently takes place in a second step. In this regard U.S. Pat. No. 7,227,359 B2 discloses a method that is based on phase gradients in the image data. However, such approaches can be prone to the occurrence of artifacts (singularities, for example) in the resulting images that hinder a consistent evaluation.
Additional examples for MR imaging methods that use phase information are known as Dixon methods. In these image data of the examination subject are acquired at different echo times. The pulse sequences are conventionally selected so that the spins of different spin species are parallel or anti-parallel at the echo times in order to enable a computational processing. However, this requirement can lead to the situation that high demands are placed on the RF system and its controller. An MR data acquisition can also take an undesirably long period of time in order to detect echo signals with the predetermined phase relationships between different spin species. A long duration of the MR data acquisition in turn increases the risk of movement artifacts.